The present invention regards a voltage converter circuit with self-oscillating half-bridge configuration and with protection against hard switching.
As is known, in all applications requiring conversion of a DC or low frequency AC voltage into an AC voltage having a higher frequency, for example, lighting applications in which the mains voltage with a frequency of 50 Hz is converted into a voltage with a frequency of 30-50 kHz for driving fluorescent or halogen lamps, voltage converter circuits are used generally having a self-oscillating half-bridge configuration.
According to a known solution, a voltage converter circuit 1 having a self-oscillating half-bridge configuration is shown in FIG. 1, and comprises first and second input terminals 2a, 2b (the second input terminal 2b being connected to ground), between which an input voltage Vin is supplied, and first and second output nodes 3a, 3b, between which an output voltage Vout is supplied. A capacitive divider 4 is connected between the pair of input terminals 2a and 2b and comprises a first capacitor 5, having a capacitance C1, and a second capacitor 6, having a capacitance C2 connected in series.
Between the input terminals 2a, 2b are also connected first and second power switches 7, 8 forming the two branches of the bridge. In particular, the first power switch 7 is connected between the first input terminal 2a and the first output node 3a (also referred to as xe2x80x9cmiddle pointxe2x80x9d), and the second power switch 8 is connected between the first output node 3a and the second input terminal 2b. 
In addition, between the first output node 3a and the second output node 3b is connected a resonant load 10 comprising a lamp 12 connected in parallel to a capacitor 13 and in series to an induction coil 14.
Each of the power switches 7, 8 has a respective control terminal 17, 18 connected to output terminals of an integrated circuit 15 controlling, in phase opposition, the opening or closing of the power switches 7, 8. In particular, when the integrated circuit 15 controls closing of the first power switch 7 and opening of the second power switch 8, the first output node 3a is connected to the first input terminal 2a; instead, when the integrated circuit 15 controls opening of the first power switch 7 and closing of the second power switch 8, the first output node 3a is connected to the second input terminal 2b. In this way, an alternating output voltage Vout is obtained at a frequency determined by switching of the switches 7, 8 and is controlled by the integrated circuit 15.
Voltage converter circuits are moreover known using discrete circuits for controlling opening and closing of power switches 7, 8. In particular, FIG. 1b is a schematic representation of a voltage converter circuit 100 comprising first and second oscillating circuits 101, 102, and a transformer 103. The first and second oscillating circuits 101 and 102 and the transformer 103 drive opening or closing of the power switches 7, 8 to generate the oscillations of the voltage supplied to the load. More specifically, the first oscillating circuit 101 is connected in parallel to the first power switch 7 and is triggered by means of a first secondary winding 104. Likewise, the second oscillating circuit 102 is connected in parallel to the second power switch 8 and is triggered by means of a second secondary winding 105. The secondary windings 104, 105 are connected to the transformer 103. A DIAC device 106 is connected to the second power switch 8 and is used to initiate the voltage converter circuit 100.
FIG. 1c shows another known voltage converter circuit, designated by 200 and comprising an oscillating circuit 201 and a driving block 203 for controlling opening or closing of the power switches 7, 8. The oscillating circuit 201 is connected to the first power switch 7 and is triggered by means of a secondary winding 202, whilst the driving block 203 is directly connected to the second input terminal 2b and to the second switch 8, and is connected to the first power switch 7 by means of a level shifter 204. A DIAC device 206 is connected to the second power switch 8 and is used to initiate the voltage converter circuit 200.
FIG. 1d shows a further known voltage converter circuit, designated by 300 and comprising a first driving circuit 301 connected to the first power switch 7 and a second driving circuit 302 connected to the second power switch 8. Both driving circuits 301, 302 are triggered by means of a respective secondary winding 303, 304. The secondary windings 303, 304 are connected to a saturable core transformer 305, which in turn is connected to a resonant load 306 by means of a winding 307. Also in this case, to initiate the voltage converter circuit 300 a DIAC device 308 connected to the second power switch 8 is used.
In order to operate correctly, the known solutions described above must meet the following two conditions:
they must not have the power switches switched on simultaneously; namely,
they must have a zero voltage condition across the power switches at the moment in which they switch on (zero voltage switching condition). In this way, the switches are prevented from dissipating a high power when they switch on (xe2x80x9chard switchingxe2x80x9d).
In particular, the latter condition is satisfied by appropriately delaying switching on of the power switches. In this connection, switching off of the second power switch 8 generates a positive variation in the value of the voltage present on the output node 3a. This voltage, after a rise time Tr, depending on the value of the current flowing in the induction coil 14 and on the equivalent capacitance present on the output node 3a, assumes the value of the voltage present on the first input terminal 2a. Consequently, to satisfy the zero voltage switching condition, it is necessary to delay switching on of the first power switch 7 by a time at least equal to the rise time Tr. In a similar way, switching off of the first power switch 7 generates a negative variation in the value of the voltage present on the output node 3a. The latter voltage, after a fall time Tf, depending on the value of the current flowing in the induction coil 14 and on the value of the equivalent capacitance present on the output node 3a, assumes the value of the voltage present on the second input terminal 2b. Also in this case, then, to satisfy the zero voltage switching condition it is necessary to delay switching on of the second power switch 8 by a time at least equal to the fall time Tf.
In the voltage converter circuit of FIG. 1a, the delay is obtained by inserting a timing circuit inside the integrated circuit 15 (plus a few components outside the integrated circuit), whereas in the voltage converter circuits of FIGS. 1b, 1c and 1d, the delay is normally obtained by means of an RC type network.
These known solutions present, however, the drawback of generating a fixed delay which is independent of the plot of the voltage present on the output node 3a. This means that if there is a change in the values of the capacitances C1 and C2, upon which the value of the equivalent capacitance present on the output node 3a depends, and/or there is a change in the value of the inductance associated to the induction coil 14, the zero voltage switching condition might no longer be respected.
According to an embodiment of the present invention, a voltage converter circuit is provided, which overcomes the limitations and drawbacks referred to above.
The voltage converter circuit has first and second input terminals; and first and second output nodes; a first power switch connected between the first input terminal and the first output node; a second power switch connected between the first output node and the second input terminal; a first delay circuit having first and second terminals connected between the first input terminal and a control terminal of the first power switch; and a second delay circuit having first and second terminals connected between the first output terminal and a control terminal of the second power switch. Each delay circuit detects a variation in the voltage supplied on the respective first terminal and detects an operating condition of the respective power switch on the second terminal, and supplies to the control terminal of the respective power switch a switching on delay signal.
A method of operation of the voltage converter circuit is also provided, according to an embodiment of the invention. The method includes delaying the closing of the first power switch in the event that the voltage at the first output node is not constant, or in the event that the second power switch is closed. The method also includes delaying the closing of the second power switch in the event that the voltage at the first output node is not constant, or in the event that the first power switch is closed.